This invention relates to complex oxide catalysts and process for producing (meth)acrolein and (meth)acrylic acid. More particularly, the invention relates to Mo-Bi-containing complex oxide catalysts which are suitable for use in production of (meth)acrolein and (meth)acrylic acid, and process for producing (meth)acrolein and (meth)acrylic acid by vapor-phase oxidation of at least one compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether, in presence of one of said catalysts.
Many proposals have been made for improved catalysts for vapor phase catalytic oxidation of propylene, isobutylene and the like to produce (meth)acrylic acid and (meth)acrolein. For example, Official Gazette of Sho 50 (1975)-13308A1-JP and Sho 50-47915A1-JP proposed catalysts containing at least an element selected from K, Rb and Cs as one of the essential components, besides Mo, Bi, Fe, Sb and Ni, and Sho 64 (1989)-56634A1-JP proposed those containing at least an element selected from Ni and Co as the essential component, in addition to Mo, Bi and Fe. Also Sho 56 (1981)-52013B1-JP disclosed catalysts containing at least an element selected from Mg, Ca, Zn, Cd and Ba as the essential component, besides Mo, Bi and Fe; and Sho 56-23969B1-JP, catalysts containing at least an element selected from IIA and IIB Group elements as the essential component, besides Mo, Bi and Fe.
In those prior art catalysts, as Bi-supplying sourse water-soluble compounds thereof, in particular, nitric acid salts are used. However, use of bismuth nitrate as the Bi-supplying source in production of the catalysts of high Bi content (e.g., those in which the atomic ratio of Mo to Bi is, where Mo is 12, Bi is 3-7) is problematical. With the view to solve that problem, Sho 62 (1987)-234548A1-JP has proposed a production method of complex oxide catalysts for which bismuth oxide or bismuth oxycarbonate are used as the Bi-supplying source.
While prior art Mo-Bi-containing complex oxide catalysts are considered to have overcome the problem incidental to the use of bismuth nitrate, there still remain shortcomings which must be improved. More specifically, for example, {circumflex over (1)} in the occasions of producing (meth)acrolein and (meth)acrylic acid by vapor-phase oxidation of propylene, isobutylene and the like using those catalysts, yield of the object products is not necessarily satisfactory, and {circumflex over (2)} due to sublimation of the molybdenum component in the catalysts during the vapor-phase oxidation reaction of propylene, isobutylene and the like, the catalytic activity decreases and the catalysts are unsatisfactory in respect of their life.
One of the objects of the present invention is, therefore, to provide complex oxide catalysts for production of (meth)acrolein and (meth)acrylic acid, which excel in activity, selectivity and catalyst life and exhibit stable performance over prolonged period.
Another object of the invention is to provide a process for producing (meth)acrolein and (meth)acrylic acid at high yield and with stability, by vapor-phase oxidation of at least one compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas in the presence of the above complex oxide catalyst.
We have made concentrative studies on complex oxide catalysts to discover that the amount of molybdenum sublimation during vapor phase oxidation reaction of propylene, isobutylene and the like is inhibited when the amounts of bismuth, iron, cobalt and nickel are relatively large. However, it is also discovered that the relatively large amounts of bismuth, iron, cobalt and nickel increase also the amount of nitrate anions at the time of catalyst preparation, which has such adverse effects as deteriorating moldability of catalyst compositions and impairing their catalytic performance. We continued further investigations with the view to solve this problem, to find that the catalysts excelling in activity, selectivity and life can be produced with good reproducibility by reducing the molar ratio of nitrate anions to molybdenum at the time of the catalyst preparation.
Thus, according to the present invention, complex oxide catalysts are provided, which are represented by general formula (1) below:
MoaWbBicFedAeBfCgDhEiOxxe2x80x83xe2x80x83(1)
(wherein Mo is molybdenum; W is tungsten; Bi is bismuth; Fe is iron; A is at least an element selected from nickel and cobalt; B is at least an element selected from sodium, potassium, rubidium, cesium and thallium; C is at least an element selected from alkaline earth metals; D is at least an element selected from phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic, boron and zinc; E is at least an element selected from silicon, aluminium, titanium and zirconium; and O is oxygen; a, b, c, d, e, f, g, h, i and x denote the atomic ratios of Mo, W, Bi, Fe, A, B, C, D, E and O, respectively; and
where a is 12, 0xe2x89xa6bxe2x89xa610, 0 less than cxe2x89xa610 (preferably 0.1xe2x89xa6cxe2x89xa610), 0 less than dxe2x89xa610 (preferably 0.1xe2x89xa6dxe2x89xa610), 2xe2x89xa6exe2x89xa615, 0 less than fxe2x89xa610 (preferably 0.001xe2x89xa6fxe2x89xa610), 0xe2x89xa6gxe2x89xa610, 0xe2x89xa6hxe2x89xa64 and 0xe2x89xa6ixe2x89xa630, and x is determined by degree of oxidation of each of the elements),
said catalysts being characterized in that the molar ratio of total nitrate anions to molybdenum at the time of catalyst preparation is adjusted to be more than 1 but not more than 1.8.
According to the invention, furthermore, there is provided a process for producing (meth)acrolein and (meth)acrylic acid through vapor-phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas, in the presence of an oxidation catalyst, the process being characterized in that it uses an above-defined complex oxide catalyst.
The catalysts of the present invention are complex oxide catalysts, at the time of whose preparation the ratio of the total molar amount of nitrate anions [NO3] to the molar amount of molybdenum [Mo] is adjusted to be more than 1 but not more than 1.8, i.e., 1 less than [NO3]/[Mo]xe2x89xa61.8. Preferably the ratio is within the range of 1.1xe2x89xa6[NO3]/[Mo]xe2x89xa61.8.
xe2x80x9cTotal molar amount of nitrate anions at the time of the catalyst preparationxe2x80x9d signifies the sum of the molar amount of nitrate anions contained in all of the starting materials which are used for preparation of the catalyst plus the molar amount of nitrate anions originating from the nitric acid which is used for the catalyst preparation as necessity arises. For example, referring to the later appearing Example 1, it is the total sum of the molar amount of nitrate anions derived from the cobalt nitrate, nickel nitrate, etc., which were used as the starting materials, plus the molar amount of the nitrate anions derived from the nitric acid which was used for dissolving bismuth nitrate.
When the ratio of [NO3] to [Mo] exceeds 1.8, deterioration in moldability and degradation in the catalytic performance result, and the objects of the present invention cannot be accomplished. That is, when such a large amount of nitrate anions is present at the time of the catalyst preparation, pH becomes very low and under the strongly acidic condition, stability and reactivity of molybdenum, tungsten and the like are adversely affected. Furthermore, because the powder obtained through such steps as evaporation to dry solid, drying and grinding contains the large amount of nitrate anions, it absorbs moisture during molding and is apt to invite deterioration in moldability. Whereas, when the ratio of [NO3] to [Mo] is 1 or less, reactivity among the used elements drops, which results in reduction in catalytic activity.
For controlling the ratio of the total molar amount of nitrate anions to that of molybdenum to be more than 1 but not more than 1.8 at the time of catalyst preparation, for example, basic bismuth nitrate is used as at least a part of the bismuth source. xe2x80x9cBasic bismuth nitratexe2x80x9d is, while its chemical formula is not necessarily established, also called, for example, bismuth oxynitrate or bismuth oxyhydroxide and has a lower nitrate anions content than that of bismuth nitrate (Bi(NO3)3xe2x80xa25H2O). For keeping the ratio within the specified range, furthermore, as the sources of other elements, compounds other than nitrates (e.g., compounds containing no or less nitric acid, such as hydroxides, carbonates, acetates or sulfates) may be used.
Of the catalysts of the present invention, those of high bismuth, iron and A component (nickel and/or cobalt) contents are preferred, because they can better reduce sublimation of molybdenum during the oxidation reaction. More specifically, referring to the general formula (1), those complex oxide compositions in which 9xe2x89xa6c+d+e, in particular, 9xe2x89xa6c+d+exe2x89xa620, are preferred. Hence it is convenient to use, as at least a part of the bismuth-supply source, basic bismuth nitrate, and as the supply sources of iron and A component, compounds containing no or less nitric acid, to render the molar ratio of total nitrate anions to molybdenum not more than 1.8 at the time of preparing the catalyst. Specifically, use of iron hydroxide or the like as the source of iron, and nickel carbonate, nickel acetate, cobalt acetate or the like as the source of A component, is preferred.
The catalysts of the present invention can be prepared by the generally practiced methods for preparing this type of catalysts, from generally used starting materials.
As the starting materials, compounds which produce oxides upon calcination, for example, ammonium salts, nitrates and the like can be used. As the method of preparation, it normally comprises dissolving or dispersing each prescribed amount of starting materials containing the elementary components in an aqueous medium, heating the solution or dispersion under stirring, then evaporating the system to dry solid, drying and grinding the solid and molding the resultant powder into optional form by extrusion molding, making tablets or granulation. In that occasion, inorganic fibers such as glass fiber and various kinds of whiskers, which are generally well known for their effect of improving strength and attrition resistance of catalyst may be added. Also for controlling the catalyst properties with good reproducibility, additives generally known as powder binder such as ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like may be used.
The catalyst of the present invention can be used by itself or may be supported on inert carriers such as alumina, silica-alumina, silicon carbide, titanium dioxide, magnesium oxide, aluminium sponge and the like.
The complex oxide catalysts according to the present invention can be obtained by calcining the molded products or those supported on carriers at 300-600xc2x0 C. for around 1-10 hours in an air stream.
The complex oxide catalysts of the present invention are favorably used for producing acrolein and acrylic acid by vapor-phase oxidation of propylene; methacrolein and methacrylic acid by vapor-phase oxidation of isobutylene; methacrolein and methacrylic acid by vapor-phase oxidation of t-butanol; and methacrolein and methacrylic acid by vapor-phase oxidation of methyl-t-butyl ether. Needless to say, furthermore, the present invention also covers such an embodiment, taking an example in the vapor-phase oxidation of propylene, of producing mainly acrolein.
Apparatus and operation conditions for practicing the vapor-phase catalytic oxidation reaction of the present invention are subject to no critical limitation. As the reactor, any of generally used fixed bed, fluidable bed and mobile bed reactors can be used, and as the reaction conditions those generally used for producing (meth)-acrolein and (meth)acrylic acid by vapor-phase catalytic oxidation can be adopted. For example, a gaseous mixture comprising 1-15 vol. % of at least one compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether as the starting gas and 1-10 vol. times of the starting gas of molecular oxygen and inert gas which is to serve as a diluent (e.g., nitrogen, carbon dioxide, steam, etc.) is contacted with a catalyst of the present invention at temperatures ranging from 250 to 450xc2x0 C., under a pressure ranging from 0.1 to 1.0 MPa and at a space velocity ranging from 300 to 5000 hrxe2x88x921 (STP) to carry out the intended reaction.
According to the process of the present invention, acrolein and acrylic acid are produced from propylene; methacrolein and methacrylic acid, from isobutylene; methacrolein and methacrylic acid, from t-butanol; and methacrolein and methacrylic acid, from methyl-t-butyl ether, at high yield.
The catalysts of the present invention can be prepared with good reproducibility, have high activity levels and exhibit high yields. Thus, according to the process of the present invention using the catalysts, (meth)acrolein and (meth)acrylic acid can be produced at high yields with high stability over prolonged period.